Spinocerebellar ataxia type 13 (SCA13) is a dominant human disease characterized by locomotor problems and substantial volume loss in the cerebellum. SCA13 is caused by mutations in the KCNC3 gene, which encodes the voltage-gated K+ channel, Kv3.3. Two allelic forms of SCA13 have been described. One form emerges in adulthood and is characterized by progressive ataxia and cerebellar degeneration. The other form is evident in infancy and is characterized by severe locomotor problems, mental retardation, and cerebellar malformation. Thus, mutations in Kv3.3 are associated with both developmental and neurodegenerative phenotypes. Due to their specialized gating properties, Kv3 channels confer on neurons the ability to fire action potentials at high frequencies. Kv3 channels also control spike duration and thereby regulate activity- dependent Ca2+ influx. The two allelic forms of SCA13 are caused by different KCNC3 mutations that alter channel activity in distinct ways. The long term goal of this research is to test the hypotheses that SCA13 mutations alter the excitability of cerebellar neurons and do so in different ways, and that these changes in excitability affect the age of onset and lead to the locomotor deficits and changes in cerebellar structure that characterize the disease. The Specific Aims of this proposal are to test the hypotheses that: 1) SCA13 mutations differentially alter the excitability of cerebellar neurons, 2) SCA13 mutations differentially alter cytoplasmic Ca2+ load in response to electrical stimulation and affect neuronal survival in Ca2+- and age- dependent cell death paradigms, and 3) the unique gating properties of Kv3 channels play a previously unsuspected role in neuronal development. These Specific Aims will be accomplished using cerebellar neurons in vitro and a vertebrate model organism, the zebrafish Danio rerio, for electrophysiological, optical, genetic, and behavioral analysis. SCA13 is rare, but analysis of SCA13 disease mechanisms may shed light on the etiology of common neurodegenerative diseases such as Alzheimer's. Given the fact that mutations in K+ and Ca2+ channel genes lead to progressive neuronal cell death in SCA13 and SCA6, it is reasonable to suggest that changes in channel function or expression contribute to susceptibility or etiology in common neurodegenerative diseases. If changes in excitability contribute to neuronal cell death, the possibilities for prevention and treatment of neurodegenerative diseases would be greatly expanded because drugs that target specific channels and modulate excitability exist and continue to be discovered.